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Topic: Wire library enables internal pull-ups -> bad for 3.3V I2C devices. (Read 2 times) previous topic - next topic

Nick Gammon

There would be a bit of a danger if a 3.3V I2C device lets the line go high (by going high impedance) the line will only reach 3.3V if you pull-up to 3.3V and not 5V. Thus, although the Atmega running at 5V will read that as a high, it is only just (it registers high at above 2.7V) - page 412 of datasheet. You would need a reasonably powerful (ie. low) pull-up resistor to make sure it reached that level quickly enough.
http://www.gammon.com.au/electronics

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